A. Field of the Invention
The device of the present invention generally relates to a modification for an electrical discharge wire cutting machine; and more particularly, to a new and improved current pick-up for conducting current pulses from a power supply into a wire electrode of an electrical discharge wire cutting machine.
B. Description of the Prior Art
Electrical discharge wire cutting machines remove material from a workpiece by spark erosion as a wire electrode is fed from a spool or similar supply through the workpiece. During a cutting operation, it is necessary to deliver current to the moving wire electrode. To insure good electrical conduction between a power supply and a moving wire electrode, a current pick-up is used. Delivery of current pulses from a power supply to a wire electrode just above and below the cutting zone is provided by actual physical contact between the wire electrode and the current pick-up.
During a cutting operation, the wire electrode is moving at approximately 200 mm/min. across the inner peripheral surface of a central bore in the current pick-up. The inner peripheral surface of the current pick-up is worn by a combination of electrical and mechanical forces. To extend the useful life of the high wear current pick-up, it is typically constructed of an extremely hard and high wear material such as carbide. Despite its extreme hardness, carbide does wear and a wire electrode passing through the carbide current pick-up ultimately erodes a slot in the central bore or wire opening of the current pick-up to a sufficient extent that electrical conduction between the carbide current pick-up and the wire electrode is detrimentally affected.
The erosion of the central bore of the current pick-up enlarges the bore making it possible for the wire electrode to be centered temporarily in the bore out of contact with the current pick-up. When this occurs, there is no electrical contact between the wire electrode and the carbide current pick-up. To prevent the wire electrode from moving out of contact with the pick-up, guides for guiding the wire electrode through the bore of the carbide current pick-up are used. The center lines of these guides are offset relative to the centerline of the bore of the carbide current pick-up causing the wire electrode to bear against the inner surface of the bore of the current pick-up.
It is believed that although delivery of current is improved by the offsetting of the axis of the guides relative to the central bore of the current pick-up, the wire electrode is slightly deformed as a result of this displacement. This deformation is caused, at least in part, by rubbing of the wire electrode over an edge where a tapered inlet and outlet merge with the bore. The deformation of the wire electrode is believed to be caused by work hardening of one side of the wire electrode surface due to engagement between the wire electrode and the inner peripheral surface of the longitudinal bore of the current pick-up and the edges in the bore. This deformation can affect the cutting accuracy of the electrical discharge wire cutting machine since deformation of the wire electrode results in a "bowing" of the wire electrode in the cutting region. This "bowing" produces a slight offset from the desired cutting path with a corresponding reduction in wire cutting accuracy.